PHYSIOGRAPHY OF NORTHWESTERN CHINA. 261 
soft alluvium. Its relation to the ancient valley of the Fén-ho, from 
which that stream was diverted in consequence of the upwarp, and its 
value as an evidence of the very recent activity of mountain growth in 
this region, have been stated. 
The F6én-ho epoch is the latest of the physiographic stages of north- 
western China. Its features belong to the immediate past and the present. 
That active mountain growth by which it is characterized continued to 
a geologic yesterday; whether it is active or quiescent to-day or will be 
in a geologic to-morrow is indeterminable. 
CORRELATION OF PHYSIOGRAPHIC STAGES. 
The position of the successive stages in the geologic time scale is 
debatable. If we reckon from the latest deformation the epochs to be 
considered are: 
(1) The Pei-t’ai, comprising much or all of the time following the 
Post-Paleozoic folding, and sufficing for complete erosion of the moun- 
tain heights due to development of the great anticlinoria and synclinoria 
noted both in northern and central China. ‘The resulting surface was one 
of advanced old age, and the cycle of erosion was unquestionably long. 
(2) The T’ang-hién epoch, not so prolonged as the Pei-t’ai, yet long 
enough to produce a warped and faulted surface and ‘to reduce it to 
advanced maturity, with surviving relief of 500 to 1,000 feet, 150 to 300 
meters. 
(3) The Hin-chéu epoch of aggradation, involving climatic change and 
extensive degradation in central Asia. 
(4) The F6n-ho epoch of mountain growth, which has resulted in 
differences of elevation of 7,500 to 10,000 feet, 2,000 to 3,000 meters, or 
more. 
The following is a provisional assignment of these epochs to geologic 
periods on the evidence now available. 
The cessation of Post-Paleozoic deformation can not be positively 
fixed; it may be early or middle Mesozoic—not later than the middle of 
the Jurassic. Beginning then, the Pei-t’ai cycle extended through the 
Cretaceous and probably well into the Tertiary; the T’ang-hién cycle may 
be assigned to early and middle Tertiary, including probably also part of 
the Pliocene; the climatic change and aggradation of Hin-chéu time fol- 
lowed during later Pliocene, and the succeeding Fén-ho epoch covers the 
Quaternary. 
